Gas Evolution Equations (Simplified): Videos & Practice Problems

A gas evolution reaction occurs when carbon dioxide gas is produced from reactants, typically involving hydrogen ions (H⁺) and bicarbonate ions. The intermediate product, carbonic acid (H₂CO₃), loses a water molecule, resulting in CO₂ gas. This process illustrates the transformation of medium products into final products, emphasizing the importance of understanding acid-base reactions and the role of water in gas-evolution equations. The reaction can be represented as:

$H^2\mathrm{CO}(aq)\to \mathrm{CO}(g)+H+(aq)$

Gas Evolution Equations (Simplified) Concept 1

A gas evolution equation describes a chemical reaction that produces carbon dioxide gas (CO₂) as a result of the transformation of certain reactants. In this context, the term "medium product" refers to an intermediate product that forms before it fully converts into the final product. Specifically, in gas evolution reactions involving acids and bicarbonate ions, the medium product is carbonic acid (H₂CO₃).

When hydrogen ions (H⁺) react with bicarbonate ions (HCO₃^-), they form carbonic acid. However, carbonic acid is unstable and cannot exist as a stable product; it quickly decomposes by losing a water molecule (H₂O). This process can be summarized by the equation:

H⁺ + HCO₃^- → H₂CO₃ → CO₂ (gas) + H₂O (liquid)

Similarly, when H⁺ reacts with carbonate ions (CO₃^2-), carbonic acid is again formed as the medium product, which subsequently decomposes to yield carbon dioxide gas and water:

H⁺ + CO₃^2- → H₂CO₃ → CO₂ (gas) + H₂O (liquid)

In essence, the gas evolution equation illustrates how carbonic acid, while initially formed, rapidly breaks down to produce carbon dioxide gas and water, highlighting the dynamic nature of these chemical reactions.

Gas Evolution Equations (Simplified) Example 1

In this example, we explore the reaction between sodium carbonate and hydrobromic acid, focusing on predicting whether a reaction occurs and writing the balanced molecular equation. The first step involves breaking down the reactants into their ionic forms. Sodium carbonate consists of sodium ions (Na⁺) and carbonate ions (CO₃^2-), while hydrobromic acid is made up of hydrogen ions (H⁺) and bromide ions (Br^-).

Next, we apply the principle of ionic partner swapping, where opposite charges attract. The sodium ion (Na⁺) pairs with the bromide ion (Br^-), forming sodium bromide (NaBr). For the carbonate and hydrogen ions, since their charges differ, we crisscross the charges to form carbonic acid (H₂CO₃). According to solubility rules, sodium bromide is soluble in water, while carbonic acid is a transient product that will decompose.

In the third step, we identify the decomposition of carbonic acid, which breaks down into water (H₂O) and carbon dioxide (CO₂) gas. This step is crucial as it highlights the gas evolution aspect of the reaction. Finally, we balance the molecular equation. Initially, we have two sodium ions on the reactant side, so we place a coefficient of 2 in front of NaBr. The carbon and oxygen atoms balance out, and we adjust the hydrogen and bromine counts by placing a coefficient of 2 in front of HBr. The final balanced equation reflects the conservation of mass and charge:

2 Na₂CO₃ + 2 HBr → 2 NaBr + H₂O + CO₂

This balanced equation illustrates the complete reaction, confirming that a reaction occurs and detailing the products formed, including the gaseous carbon dioxide.

Predict the products formed from the following gas evolution equation.

• A. KCl, H₂CO₃
• B. CO₂, H₂O
• C. KCl, CO₂, H₂O
• D. KCl, H₂, CO₃^2–

Answer: (C)

Here’s what students ask on this topic:

What is a gas evolution reaction in chemistry?

A gas evolution reaction in chemistry is a type of chemical reaction where a gas is produced as one of the products. In the context of acid-base reactions, this often involves the reaction of hydrogen ions (H⁺) with bicarbonate ions (HCO₃^-) to form carbonic acid (H₂CO₃). The carbonic acid then decomposes into carbon dioxide gas (CO₂) and water (H₂O). This can be represented by the equation:

$H_2{\mathrm{CO}}_3\to {\mathrm{CO}}_2\left(g\right)+H_2\left(l\right)$

How is carbon dioxide gas formed in a gas evolution reaction?

In a gas evolution reaction, carbon dioxide gas (CO₂) is typically formed when hydrogen ions (H⁺) react with bicarbonate ions (HCO₃^-). This reaction produces carbonic acid (H₂CO₃) as an intermediate product. Carbonic acid is unstable and quickly decomposes into carbon dioxide gas and water. The overall reaction can be summarized as:

$H_2{\mathrm{CO}}_3\to {\mathrm{CO}}_2\left(g\right)+H_2\left(l\right)$

What is the role of water in gas evolution reactions?

In gas evolution reactions, water plays a crucial role in the decomposition of the intermediate product, carbonic acid (H₂CO₃). When carbonic acid forms, it is unstable and quickly breaks down into carbon dioxide gas (CO₂) and water (H₂O). The water is not lost but is part of the final products of the reaction. This process can be represented by the equation:

$H_2{\mathrm{CO}}_3\to {\mathrm{CO}}_2\left(g\right)+H_2\left(l\right)$

What are intermediate products in gas evolution reactions?

Intermediate products in gas evolution reactions are temporary compounds formed during the reaction process before the final products are produced. In the case of reactions involving hydrogen ions (H⁺) and bicarbonate ions (HCO₃^-), the intermediate product is carbonic acid (H₂CO₃). This intermediate is unstable and quickly decomposes into carbon dioxide gas (CO₂) and water (H₂O). The reaction can be summarized as:

$H_2{\mathrm{CO}}_3\to {\mathrm{CO}}_2\left(g\right)+H_2\left(l\right)$

Why is carbonic acid considered unstable in gas evolution reactions?

Carbonic acid (H₂CO₃) is considered unstable in gas evolution reactions because it readily decomposes into carbon dioxide gas (CO₂) and water (H₂O). This instability is due to the weak nature of carbonic acid, which cannot be isolated under normal conditions. As soon as it forms, it breaks down into CO₂ and H₂O, driving the reaction forward and producing the gas. The decomposition can be represented by the equation:

$H_2{\mathrm{CO}}_3\to {\mathrm{CO}}_2\left(g\right)+H_2\left(l\right)$